The present invention relates to the repair of composite structures, and particularly, to improved methods and apparatus for applying heat to repair sites for the curing of such repair.
Composite structures of laminated material are increasingly being used in industry, and particularly, in the aircraft industry. From time to time, the composite structure will be damaged and need repair. Repair typically involves the removal of damaged material and covering the repair site with layers of woven material, such as graphite, which have been impregnated with mixed epoxy/resin. The new material and the adhesive are then pressed into place and heat cured. Pressure conforms the new material to the existing material and heat curing properly sets the resin or adhesive. When correctly done, heat curing involves a controlled ramp-up to a predetermined temperature which is held for a period of time followed by a controlled ramp-down to a lower temperature.
Controlling the accuracy of pressure, temperature and time for a repair is important to the strength of repair. Improper temperature control can substantially impact repair strength. A temperature ramp up, which is not properly controlled can shock and weaken the composite structure being repaired. Curing temperatures lower than desired result in poor bonding and temperatures higher than desired can result in burning both the repair patch and the material being repaired. A combination of these effects can be found at a repair site which is subjected to uneven temperatures.
Heat can be applied to a repair site by placing electrical resistance heating blankets directly over the entire repair patch area. When known heating blankets are laid over a repair site, they tend to heat the site unevenly, making improper curing possible. In one study, an aircraft skin was heated by direct contact with a heating blanket and a 95.degree. F. temperature variation was detected under the blanket when the skin was heated to a nominal 360.degree. F. In a second study, in which a copper foil heat conductor was inserted between the heating blanket and the aircraft skin, a 65.degree. F. variation was found. Proper curing is not possible with such a large temperature variation.
The difficulties of achieving even heating are compounded by uneven thermal characteristics at a repair site. For example, a portion of a repair site may consist of aircraft skin only, while another portion may consist of aircraft skin having an underlying support member. The support member tends to cool the skin to which it is attached causing uneven temperatures across the repair site. Multi-zone heating blankets have been used to provide different amounts of heat to different portions of a repair site in an attempt to provide even temperatures to areas having varying thermal characteristics. Such multi-zone heating blankets however, require complex control systems for the multiple heating zones of the blanket, and importantly, must be specifically configured to provide heating characteristics corresponding to the thermal characteristics of different areas of the repair site. Providing complex controls and many different configurations of multi-zone heating blankets is a problem for repair centers.
Another system for repairing composite structures is disclosed in United States Patent 4,808,253 to Mimbs. The Mimbs system discusses placing a heating blanket on top of a thermally conductive fluid-filled envelope and distributing heat and pressure to a repair site through the envelope. As disclosed, the envelope is primarily used to conform the repair assembly to contoured shapes. The Mimbs system does not result in a sufficiently accurate temperature control. Overlaying the envelope with a conventional heating blanket creates multiple heat transfer interfaces resulting in inefficient and inconsistent heat transfer to the fluid in the envelope. The heating inefficiencies can result in inability to achieve either an accurate heat ramp-up or actual curing temperatures in the order of 350.degree. F. Additionally, no means of measuring heat ramp-up or curing temperature is shown by the Mimbs Patent.
A need exists for composite repair apparatus which can achieve accurate heat ramp-up and sufficiently uniform temperatures at a repair site to achieve proper bonding between patch material and the structure being repaired.